FIG. 1 illustrates amplifying circuitry 100 comprising a differential amplifying circuit 101. Circuit 101 comprises a differential pair of transistors 102, 104, receiving at their gates differential input voltages VIN1 and VIN2 respectively. Transistor 102 of the differential pair is for example a MOS transistor having its drain coupled to a high supply voltage VDD via a resistor 106, while transistor 104 is for example a MOS transistor having its drain coupled to VDD via a resistor 108. Transistors 102, 104 have their sources coupled together by a line 110. A current source 112 is provided between the sources of transistors 102, 104 and ground, and comprises first and second transistors 112A and 112B.
The differential amplifying circuit 101 provides differential output signals VOUT2 and VOUT1 on output lines 114 and 116 respectively coupled to the drains of transistors 102, 104. The gain of the amplifier is determined by the trans-conductance “GM” of the transistors 102 and 104, and the resistance R of each of the resistors 106, 108. In particular, the gain is equal to GM×R.
During operation of the differential amplifier 100, the gain may depart from expected values due to temperature and/or process variations that cause variations in the values of GM and R. This is undesirable in many applications of the amplifier 100, as too much gain can cause linearity problems, for example the output signal swing being too large for circuit capabilities, and too little gain can cause noise problems, for example the output signal swing not being significantly larger than noise generated by the circuit.
One solution for overcoming this disadvantage would be to replace the resistors 106 and 108 by transistors. Then, variations in the transconductance GM would be compensated, leading to a more constant gain. However, a problem with such a solution is that these transistors would have a drain source voltage drop higher than the voltage drop across the resistors 106, 108. Thus to maintain the same output voltage, a higher supply voltage would be needed, which is a problem due to the general trend in the industry to reduce supply voltages.
As shown in FIG. 1, an alternative solution is to control the current source 112 such that the total current 2I through the current source 112 varies to counteract variations in the resistance R and transconductance GM. A biasing circuit 117 is used to control the current source 112, and comprises a transistor 118 coupled in series with a variable current source 120. The current I flowing through the variable current source 120 matches the current I flowing through each of the transistors 112A, 112B of the variable current source 112.
However, there is a difficulty in designing the variable current source 120 to accurately match both R and GM variations occurring in the components 102, 104, 106 and 108 of the differential amplifying circuit 101. There is thus a need for a differential amplifier having a biasing circuit that effectively controls the current source 112.